Field of the Invention
The present invention relates to a light extraction substrate for an organic light-emitting device (OLED) and a method of fabricating the same, and more particularly, to a light extraction substrate for an OLED which can improve the brightness of a display or an illumination system to which an OLED is applied by improving light extraction efficiency and a method of manufacturing the same.
Description of Related Art
In general, an organic light-emitting device (OLED) includes an anode, a light-emitting layer and a cathode. When a voltage is applied between the anode and the cathode, holes are injected from the anode into the organic light-emitting layer and electrons are injected from the cathode into the organic light-emitting layer. Holes and electrons that have migrated into the organic light-emitting layer recombine with each other in the organic light-emitting layer, thereby generating excitons. When such excitons transit from the excited state to the ground state, light is emitted.
Organic light-emitting displays including an OLED are divided into a passive matrix type and an active matrix type depending on a mechanism that drives an N*M number of pixels which are arranged in the shape of a matrix.
In the active matrix type, a pixel electrode which defines a light-emitting area and a unit pixel driving circuit which applies a current or voltage to the pixel electrode are positioned in a unit pixel area. The unit pixel driving circuit has at least two thin-film transistors (TFTs) and one capacitor. Due to this configuration, the unit pixel driving circuit can supply a constant current irrespective of the number of pixels, thereby realizing uniform luminance. In addition, the active matrix type organic light-emitting display consumes little power, and thus can be advantageously applied to high definition displays and large displays. In contrast, the passive matrix type is configured such that a current is directly applied to each pixel. Although the passive matrix type is simpler than the active matrix type, it is not suitable for a high-definition display. Therefore, the passive matrix type is generally used for small displays or illumination systems.
However, although OLEDs are in the stage of commercial distribution, light loss still occurs at the interface between the thin film layer of the device and the substrate due to the difference between refractive indexes. Accordingly, the light extraction efficiency of OLEDs is limited to about 20%, which is problematic. It is difficult for OLEDs to realize a high efficiency without improving the light extraction efficiency.
In order to overcome this problem, an approach of improving an optical efficiency by forming a light extraction layer on an OLED was proposed. In the related art, such light extraction layers were formed via photolithography. However, there are the following problems in that a cost is increased due to the use of expensive equipment and a process becomes complicated, which is problematic.
In addition, the light extraction layer was formed by patterning a polymeric plastic film and then attaching the patterned film on an OLED. However, there are limitations not only to an increase in the refractive index of the polymeric plastic material but also to the mechanical and thermal endurance of the material.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.